Research On Assessment And Regulation Methods For Power System Voltage Instability Risk

The contingencies'uncertainties and electricity market operating rules have been the big threat to the power system stability. Regarding the system instability as a risk, power system instability risk assessment and risk management have been paid much more attention by researchers and engineers of power engineering. Nowadays, risk assessment or risk management has become one of the most challenging tasks in the power engineering field.After summarizing up-to-date studies on power system risk, this thesis does some research work in power system voltage instability risk assessment, monitoring, regulation and control.Voltage instability probability is one of the fundamental factors of voltage instability risk assessment based on probability theory. After analyzing the characteristics of Thevenin equivalent parameters'tracing based on local measurement and power flow, The magnitude ratio of Thevenin equivalent impedance to load impedance is taken as voltage instability index. According to the ratio, probability density function and distribution function of the power system voltage instability are proposed. In the analysis of system contingency probability, the simple probability models of system contingency and experimental distribution of system contingency occurrence are analyzed. With the Poisson distribution, the formula to calculate ovoltage instability probability based on some operation state is presented. This voltage instability probability calculation method can decrease the workload of risk assessment notably, and the simulation results show that the method is feasible.Voltage instability impact is another fundamental factor of voltage instability risk assessment based on probability theory. With no consideration of investment capital and all kinds of electricity markets'situations, monetary impact formula of the power system voltage instability are proposed from the four different standpoints of load, generation, transmission, generation and transmission. If the voltage instability impacts are quantitated monetarily too many factors may be concernd and the monetary economy indices are hard to calculate, which makes the monetary quantitative assessment method itself has a risk. Therefore, an impetance magnitude ratio based on Thevenin equivalent is employed as a scheme to evaluate voltage instability impact.The instability risk result obtained by instability probability multiplying instability impact is usually very small, and this is apt to let people misunderstand the value and seriousness behind the risk. Therefore, it is hard to provide information that should be precise and close to the common sense to make people generally understand for risk management and control decision-making. According to these phenomena, after defining minimum attention probability level and minimum attention impact level, the minimum attention risk can be obtained by traditional risk calculation. Thus, the very smaller risk value from traditional risk calculation method is normalized, which can transform the risk into an understandable qualitative judgment scope to provide decision-making information. Then risk assessment method is applied in the power system contingencies screening and ranking, and a new contingencies screening and ranking method containing risk information is proposed.In order to provide voltage instability risk information for the dispatchers, the on-line voltage instability risk monitoring and pre-warning scheme based on whole network information and local measurements are proposed. In the scheme, the system voltage instability risk level is ranked by reserved system power information. And the system can be divided into control area, high-alert area, alert area, high-attention area and attention area with black, red, orange, yellow and green to represent the risk ranks respectively. The contribution ratios of all units to system voltage instability risk are calculated. Some units are selected according to their high ratios and risk caution information is shown also. Risk tendency forecasting is obtained by evaluating voltage instability risk based on state pre-warning results. The system risk tendency alert information is given for future high system risk level or high risk accumulating. A PMU based risk monitoring scheme is also proposed. It can employ local measurements to process local risk monitoring and system risk estimation in case that the state estimator failed. Therefore, the scheme can be complementary to risk monitoring based on whole network information.To make the voltage instability risk run in an acceptable scope, an on-line voltage instability risk control scheme and method is presented. The control is made up of generation output power regulation and load shedding/restricting. After the risk monitoring system sends risk adjustment command generation output power is regulated first, in order to avoid to losing load as much as posible. If this adjustment can reach a satisfied risk level, no load shedding/restricting should execute; if not, load shedding/restricting should execute on the base of generation output power regulation results. The regulation and control scheme can evade or decrease system voltage instability risk and could take the system economical operation into account.